How does the Vertex Weight Mix Modifier Work Internally
Have a slow read through the manual page on the vertex weight mix modifier, it has an example at the end to help explain it.

The points I think are most relevant -


  Vertex Group A
      The vertex group to affect.
  Default Weight A
      The default weight to assign to all vertices not in the given vertex group.
  Vertex Group B
      The second vertex group to mix into the affected one. Leave it empty if you only want to mix in a simple value.
  Default Weight B
      The default weight to assign to all vertices not in the given second vertex group.
  
  Mix Set
      Choose which vertices will be affected.
      All vertices
          Affects all vertices, disregarding the vertex groups content.
      Vertices from group A
          Affects only vertices belonging to the affected vertex group.
      Vertices from group B
          Affects only vertices belonging to the second vertex group.
      Vertices from one group
          Affects only vertices belonging to at least one of the vertex groups.
      Vertices from both groups
          Affects only vertices belonging to both vertex groups.
  
  Texture Mask
      An additional texture, the values of which will be multiplied with the global influence value for each vertex.


So Vertex Group A is the group that gets the result, you use this in modifiers after the mix.

The mix mode defines how the two values are mixed, while the set decides which vertices get the weight.

The texture mask can then limit the result to certain areas of the mesh.